In recent years, an FM radio receiver and/or an FM transmitter may be carried on a portable device (including an electronic device) such as a mobile phone. In this case, it is necessary for the portable device to provide a filter which allows a signal in the FM band to pass therethrough in addition to a filter which allows a signal in the ordinarily used frequency band to pass therethrough.
A dielectric filter has been hitherto used as the filter which allows the signal in the FM band to pass therethrough.
The conventional dielectric filter includes, for example, a filter disclosed, for example, in Patent Document 1. However, this filter is provided in order to attenuate the spurious at a level of 500 MHz. It is true that a signal at 76 to 108 MHz including the FM band is allowed to pass the filter. However, the filter is not for attenuating the signal in the high frequency band, for example, at 800 MHz, 1.4 GHz, 1.9 GHz, 2.5 GHz, and 5 GHz as the frequency band ordinarily used by a portable device.
Therefore, it is obvious that the noise in the high frequency band cannot be suppressed when the FM radio receiver and/or the FM transmitter is carried on the portable device.
As for the value of the inductance required for the filter, the lower the frequency is, the larger the required value is. When a coil is formed with a filter having a passband in the vicinity of 100 MHz, it is necessary to increase the number of turns and/or increase the size of the coil in order to obtain the required inductance.
The required inductance is obtained by increasing the number of turns of the coil. However, because the conductor resistance of the coil is increased, the Q characteristic is consequently deteriorated, failing to obtain suitable filter characteristic. Further, because stray components disadvantageously appear, and suitable attenuation characteristic is not obtained. Furthermore, because the area of the coil is increased, it is impossible to realize a compact size. By contrast, the portable device on which the filer is carried is required to be compact in size. In relation thereto, the dimension of the conventional filter is so large, i.e., 4.8 mm×3.5 mm, that it is not usable for the portable device.
In the conventional technique, a stacked type electronic part which has a base member obtained by joining a dielectric layer and a magnetic layer (see, for example, Patent Document 2) has been proposed. However, the stacked type electronic part is provided only to suppress warpage, delamination, and cracks in the product by adding a dummy layer. It is unclear whether or not the stacked type electronic part makes it possible to carry the FM radio receiver and/or the FM transmitter on the portable device.
Patent Document 1: Japanese Patent No. 2505135;
Patent Document 2: Japanese Laid-Open Patent Publication No. 2003-37022.